Phosphorus-containing activated carbon as acid support in a bifunctional Pt–Pd catalyst for tire oil hydrocracking

    Phosphorus-containing activated carbon as acid support in a bifunctional Pt-Pd catalyst for tire oil hydrocracking Idoia Hita, Tom´as Cordero-Lanzac, Aurora Gallardo, Jos´e Mar´ıa Arandes, Jos´e Rodr´ıguez-Mirasol, Javier Bilbao, Tom´as Cordero, Pedro Casta˜no PII: DOI: Reference:

S1566-7367(16)30036-X doi: 10.1016/j.catcom.2016.01.035 CATCOM 4580

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

29 October 2015 24 January 2016 29 January 2016

Please cite this article as: Idoia Hita, Tom´as Cordero-Lanzac, Aurora Gallardo, Jos´e Mar´ıa Arandes, Jos´e Rodr´ıguez-Mirasol, Javier Bilbao, Tom´ as Cordero, Pedro Casta˜ no, Phosphorus-containing activated carbon as acid support in a bifunctional Pt-Pd catalyst for tire oil hydrocracking, Catalysis Communications (2016), doi: 10.1016/j.catcom.2016.01.035

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ACCEPTED MANUSCRIPT Phosphorus-containing activated carbon as acid support in a

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bifunctional Pt-Pd catalyst for tire oil hydrocracking

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Idoia Hita1, Tomás Cordero-Lanzac1, Aurora Gallardo2, José María Arandes1, José

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Department of Chemical Engineering, University of the Basque Country UPV/EHU, P.O. Box 644-

48080 Bilbao, Spain. * E-mail: [email protected]

Universidad de Málaga, Department of Chemical Engineering, Faculty of Science, Campus de Teatinos,

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Rodríguez-Mirasol2, Javier Bilbao1, Tomás Cordero2, Pedro Castaño1

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s/n, 29010, Málaga, Spain

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ABSTRACT

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A bifunctional Pt-Pd catalyst supported on phosphorus-containing activated carbon has

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been prepared, characterized and tested in the hydrocracking of a hydrotreated scrap tires pyrolysis oil. The product has a very interesting composition: 48-78 wt% naphtha and 19-42 wt% diesel fractions, with moderate amounts of aromatics (< 40 wt%) and

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sulfur (< 250 ppm). The challenge was to prepare a stable, porous, selective and acid carbonaceous catalyst from waste (olive stone), what has been confirmed from the catalytic properties and product distribution point of view. In fact, phosphate groups in the activated carbon are stable hydrocracking sites, with comparable performance than that of the acid sites present in amorphous SiO2-Al2O3. Keywords: tire pyrolysis oil, acid activated carbon, bifunctional hydrocracking catalyst, naphtha, diesel, coke deactivation

ACCEPTED MANUSCRIPT 1. Introduction The sequenced pyrolysis and hydroprocessing of scrap tires offers the possibility of

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obtaining high quality fuels, which represents a great challenge for waste valorization at a great scale [1]. Flash pyrolysis allows for maximizing the liquid (scrap tire pyrolysis

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oil or tire oil) yield [2,3] which, together with adulterated carbon black is an abundant and economically interesting pyrolysis product from tires [4,5]. Tire oil has a relatively

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high calorific value but on the other hand, it also has a high content of sulfur, heavy molecules -boiling point (BP) higher than 350 ºC- and aromatics [3,6,7]. Typical

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hydrotreaters using Ni-Mo or Co-Mo catalysts are unable to upgrade the composition of tire oil up to a point that is required by the environmental legislations [8]. To this aim,

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subsequent hydrocracking contributes to further lowering of sulfur and aromatics, but

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particularly converting heavy molecules into naphtha and diesel fractions. This

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hydrocracking of the pretreated tire oil has been studied using active phases like Pt, Pd or Ir supported over different supports (zeolites, innovative mesoporous materials) [8] with similar results to those obtained on the hydrocracking of aromatic refinery streams

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like light cycle oil (LCO) [9]. Activated carbons are prepared from different precursors like low-cost wastes [10], and present good features as catalyst supports, in terms of larger specific areas, tunable surface groups, chemical stability, or easy metal recovery by combustion or gasification of the support [11-13]. Carbon materials are frequently subjected to additional treatments to increase their surface acidity by generating carboxylic groups on the surface. However, these functional groups are not thermally stable (they decompose at 250 ºC) [14]. Chemical activation with H3PO4 leads to a well-developed porous structure with thermally stable oxygen-phosphorus surface groups (mainly C-O-PO3 and

ACCEPTED MANUSCRIPT C-PO3) that provide activated carbons with higher surface acidity and turn carbon into a more oxidation-resistant material [15], namely phosphorous-containing activated carbon

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(ACP), which have proven to have a good performance as catalysts [14].

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In this work, we have synthesized, characterized and tested a cheaper and more

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environmentally friendly bifunctional catalyst (Pt-Pd supported on an ACP) for the hydrocracking of a pretreated tire oil for readjusting its composition. Fresh and spent

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catalysts have been characterized using various techniques for studying its physicochemical properties, as well as a qualitative analysis of deposited coke. Liquid reaction

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products have been analyzed by chromatographic (GCGC) technique. Our ultimate goal is preparing a bifuncitonal catalyst from agricultural waste (olive stone), with

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similar features to these of SiO2-Al2O3 based catalysts, for mitigating a pressing

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2. Experimental

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environmental problem (waste tire).

The activated carbon was obtained using olive stone as precursor, impregnating it with

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an aqueous solution of H3PO4 (85 wt%, 3 g of H3PO4 per g of precursor) at room temperature and dried at 60 ºC for 24 h. The impregnated precursor was activated in a tubular furnace under a continuous N2 flow (150 cm3 STP min-1), rising the temperature at 10 ºC min-1, up to 500 ºC for 2 h. Then the catalyst was cooled inside the furnace under a N2 flow and washed with distilled water at 60 ºC until constant pH was reached and negative phosphate presence in the eluent was achieved [16]. The obtained activated carbon was dried in a vacuum drier at 100 ºC and sieved to a particle size of 100-300 µm. Pt and Pd were incorporated on the support by simultaneous incipient wetting impregnation method, using an aqueous solution of HPtCl6·6HCl and PdCl2 slightly acidified with HCl. The impregnated support was calcined at 400 ºC for 4 h

ACCEPTED MANUSCRIPT with a N2 flow of 150 cm3 STP min-1 in a tubular furnace to yield a supported catalyst with 0.5 wt% Pd and 1 wt% Pt. This ratio of Pt:Pd have proved to be valuable for

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having a stable catalyst against the sulfur poisoning [17].

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Scanning electron microscopy (SEM) was performed in a JSM 6490LV JEOL

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microscope at 25 kV. Transmission electron microscopy (TEM) was performed in a Phillips CM200 at 200 kV. N2 adsorption-desorption isotherms was performed in

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Micromeritics ASAP2020 apparatus at -196 ºC. X-ray photoelectron spectroscopy (XPS) was carried out in a 5700 C model Physical Electronics apparatus. The acidity of

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the catalyst was studied by adsorption of tert-butylamine (t-BA) at 100 ºC, using a Setaram DSC-111 calorimeter. The temperature programed desorption (TPD) was

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performed, recording the butene signal (main cracking product) in a mass spectrometer

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(m/z = 56) and rising temperature at 5 ºC min-1 up to 500 ºC.

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The tire oil was obtained in a sequenced process: (i) pyrolysis of scrap tires in a continuous conical spouted bed reactor at 500 ºC [18], and (ii) hydrotreating in a trickle bed reactor, using the previously described conditions [6], aiming for reducing sulfur

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content and thus minimizing deactivation due to catalyst poisoning over the metallic PtPd function. Hydrocracking of hydrotreated tire oil has been performed in a trickle bed reactor under the following conditions: space time, 0-0.28 gcat h gfeed-1; 400-500 ºC; 65 bar; H2/oil ratio of 1,000 vol%; and time on stream (TOS) of 0-6 h. Prior to the reaction, the catalyst is reduced under atmospheric pressure at 400 ºC. The feed has been diluted 50 vol% in n-decane. The feed and reaction products have been analyzed offline by means of comprehensive gas chromatography (GCGC) coupled in line with mass spectrometry using both a flame ionization detector and a mass selective detector. Sulfur content has been measured using an Agilent 7890A apparatus equipped with a pulsed flame photometric

ACCEPTED MANUSCRIPT detector. The composition of the feed is: naphtha (BP = 35-216 ºC), 27.9 wt%; diesel (BP = 216-350 ºC), 50.3 wt%; and gasoil (BP > 350 ºC), 21.8 wt%. As from a lump

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classification: paraffins and isoparaffins, 35.4 wt%; naphthenes, 22.0 wt%; 1-ring

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aromatics, 31.9 wt%; and 2-ring aromatics, 10.7 wt%. As from sulfur speciation:

144 ppm;

trimethyldibenzothiophene

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methyldibenzothiophene (M1DBT), 38 ppm; dimethyldibenzothiophene (M2DBT), (M3DBT),

1,019 ppm;

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tetramethyldibenzothiophene (M4DBT), 784 ppm.

Coke deposited on the catalyst was analyzed by temperature-programmed oxidation

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(TPO), using a TA Instruments TGA Q5000 IR apparatus.

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3. Results and discussion

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During the preparation of the activated carbon, 46 wt% carbonization-activation yield is obtained, whereas H3PO4 treatments catalyzed dehydration and recombination reactions

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of the carbon radicals and favors the aromatization in the activated carbon structure [10]. Table 1 summarizes the main properties of the bifunctional Pt-Pd phosphorous-

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containing activated carbon catalyst (Pt-Pd/ACP). The catalyst has a relatively high porosity, with an average pore size of 52.3 Å, and high specific surface (1,305 m2 g-1). The P:O ratio (Table 1) demonstrate the existence of phosphate groups on the catalytic surface [19] due to the H3PO4 treatment, generating acid sites [20]. Table 1 demonstrates that the acidity of the bifunctional catalyst and its surface area are comparable to those of a silica-alumina as support [8]. SEM image of the (Figure 1a) indicates that the surface of the catalyst has a very high porosity with irregular channel shapes and sizes of about 10-30 µm whereas TEM image (Figure 1b) shows that the bimetallic Pt-Pd particles are well dispersed on the support with a homogeneous sizes of 7-15 nm.

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Fig. 1. a) SEM and b) TEM images of fresh catalyst.

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Table 1. Properties of the fresh and spent catalysts (440 ºC, space time of 0.28 gcat h gfeed-1).

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Porous structure SBET (m2 g-1) Vmicrop (cm3 g-1) Vmesop (cm3 g-1) Average pore diameter (Å) Surface and chemical analysis (XPS) C1s (wt%) O1s (wt%) P2p (wt%) Pd3d (wt%) Pt4f (wt%) Total acidity (mmolt-BA g-1)

Fresh

Spent

1305 0.512 0.829 52.3

145 0.449 106.0

88.40 8.17 2.18 0.45 0.80 0.29

91.71 6.52 1.02 0.32 0.43 0.06

The evolution with time on stream and space time of hydrocracking conversion is shown in Figure 2a. Hydrocracking conversion (XHC) has been defined as a function of the gasoil mass fraction (xGasoil) removed:

X HC 

x Gasoil STPO  x Gasoil prod x Gasoil STPO

(1)

During the 6 h on stream the catalyst deactivates until it reaches a pseudo-stationary state with very limited further decay. This situation is due to almost equal coke

ACCEPTED MANUSCRIPT formation and coke precursor hydrocracking rates [21]. Temperature has a remarkable effect on product distribution as shown in Figure 2b. Upon increasing process

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temperature, naphtha concentration increases at the expense of diesel and gasoil. A good

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compromise of product distribution is obtained at 480 ºC with contents of naphtha and

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diesel of 53 and 45 wt%, respectively (Figures 2a,b). Furthermore, the phosphate sites of the activated carbon are stable and have enough acid strength in order to promote the

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scission of the gasoil molecules contained in the tire oil.

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60

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XHC (wt%)

-1

0.28 gcat h gfeed

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20

-1

0.16 gcat h gfeed

a)

40

-1

0.05 gcat h gfeed

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0

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0

1

2

3

4

5

6

Time on stream (h)

Composition (wt%)

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80

b)

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Naphtha Diesel

Gasoil

40

20

0 Feed

400

440

480

500

Temperature (ºC)

Fig. 2. a) Effect of space time on the evolution with time on stream of hydrocracking conversion of tire oil at 440 ºC, and b) effect of temperature on product lump distribution at TOS = 6 h; space time, 0.28 gcat h gfeed-1.

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(Figure

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at

different

hydrocracking

temperatures,

where

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hydrodesulfurization conversion (XHDS) has been defined based on total sulfur content

S STPO  S prod

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X HDS 

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(S) as:

S STPO

(2)

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As shown in Figure 3a, and according to the bifunctional catalyst mechanism [22], 1 and 2-ring aromatics hydrogenate on the metallic sites of the catalyst to form

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naphthenes, which subsequently suffer ring opening to form paraffins and isoparaffins on the acid sites of the activated carbon. The first hydrogenation is a reversible

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exothermic reaction, so if the cracking activity is not dominant, the yield of aromatics

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increases with temperature (400-440 ºC), as a consequence of the thermodynamic equilibrium of hydrogenation being disfavored. At temperatures of 480 ºC and above,

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cracking and ring opening are more dominant and naphthenes are easily converted into paraffins and isoparaffins. In these conditions, paraffins and isoparaffins are the most

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abundant components, up to 50 wt%. In Figure 3b, it is observed that, as a consequence of deactivation, X HDS decreases from almost complete conversion down to values between 65-90 % after 6 h, upon increasing temperature. The pretreated tire oil contains high amounts of sulfur compounds of the M3 and M4DBT groups (>1000 and >750 ppm, respectively) that are efficiently removed at a temperature higher than 480 ºC. On the other hand, M2DBT concentration increases with temperature due to isomerization and dealkylation reactions of DBTs. The methyl groups in M2DBT tend to be placed in positions 2 and 4 where they inhibit the HDS reaction of the molecule. Nonetheless, the catalyst is able to effectively remove sulfur compounds not to surpass 250 ppm (90 wt% conversion). These results

ACCEPTED MANUSCRIPT indicate that the composition of the products is comparable to that using a Pt-Pd/SiO2Al2O3 catalyst in terms of the removal of gasoil, aromatics and sulfur [8], proving that

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ACP can be successfully used as a support for a bifunctional catalyst in this type of

Paraffins/isoparaffins Naphthenes 1-ring aromatics 2-ring aromatics

a)

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60

40

0

400

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Feed

480

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500 100

b) 80

750

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Sulfur (ppm)

1000

440

Temperature (ºC)

M1DBT M2DBT

500

60

M3DBT

XHDS (%)

20

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Concentration (wt%)

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reactions processing complex heavy feedstock.

M4DBT

250

40

0 Feed

400

440

480

500

Temperature (ºC)

Fig. 3. a) Effect of the hydrocracking temperature on product composition and b) in sulfur speciation at TOS, 6 h; space time, 0.28 gcat h gfeed-1.

It has been shown that the catalyst deactivates during the initial 6 h on stream (Figure 2a), whereas the main source of the decay is due to coke fowling [23]. Figure 4a shows the N2 adsorption-desorption isotherms of the fresh and the spent catalysts. Both isotherms have a I+IV-type adsorption shape characteristic of a well-developed microporous and mesoporous structure, with H4-type hysteresis loop typical of lamellar

ACCEPTED MANUSCRIPT mesoporous materials. The amount of N2 adsorbed by the spent catalyst is significantly lower than the fresh counterpart due to the fact that coke is blocking the micropores and

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the more narrow mesopores. As listed in Table 1, the spent catalyst suffered an

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important decrease of the specific surface (1,305 to 145 m2 g-1), micropore volume

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(0.512 cm3 g-1 to a non-measurable value) and mesopore volume (0.829 to 0.449 cm3 g). XPS analysis (Table 1) reveals an increase of the carbon concentration and a

significant decrease in the amount of accessible Pt, Pd (both components of the metallic

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function) and P (acid function). These results, together with the total micropore blocking, indicate that the remaining metallic and acid sites are located in the

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mesopores. Furthermore, total acidity also decreases from 0.29 mmol g-1 to

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0.06 mmol g-1.

a)

3

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0 0.0

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Fresh Spent

600

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-1

Vads (cm gcat )

800

0.2

0.4

0.6

0.8

1.0

Relative pressure 0.25

b) Fresh Spent

-1

DTG (µg s )

0.20 0.15 0.10 0.05

Coke combustion zone

0.00 100

200

300

400

500

600

700

Temperature (ºC)

Fig. 4. a) N2 adsorption-desorption isotherms at -196 ºC and b) TPO profiles of the fresh and spent catalysts at: TOS, 6 h; space time, 0.28 gcat h gfeed-1.

ACCEPTED MANUSCRIPT Figure 4b shows the TPO curves for the fresh and spent catalysts, where it can be seen that combustion of the coke deposited on the spent catalyst has a differentiated profile,

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starting at 250 ºC and with a well-defined peak at 360 ºC. This low temperature

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combustion is characteristic of a less developed coke, and the peak is due to the

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combustion of the coke located over the metallic sites, which act as catalysts in the combustion [24].

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4. Conclusions

Phosphorous-containing activated carbon (ACP) offers great perspectives as a cheap

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alternative for being used as acid support in bifunctional hydrocracking catalysts. Chemical activation with H3PO4 leads to a carbon support with a high specific surface

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and enhanced surface acidity due to the highly stable C-O-PO3 and C-PO3 surface

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hydrocracking reactions.

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groups. These groups are capable of taking in oxygen, and form acid sites which enable

Despite the tire oil having a complex composition of molecules of low reactivity, the PtPd/ACP bifunctional catalyst is capable of removing 97.3 % of the original sulfur and

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up to 97 wt% of the heavy gasoil molecules in the pretreated tire oil. Catalyst deactivation occurs due to coke formation on the catalytic surface, which is partially located over the metallic sites, and shows a low development degree. Acknowledgements This work was carried out with the support of the Ministry of Economy and Competitiveness of the Spanish Government, some cofounded with ERDF funds (CTQ2012-35192, CTQ2012-36408 and CTQ2013-46172-P), the Basque Government (SAIOTEK SA-2011/00098, SA-2013/00173 and IT748-13), and the University of the

ACCEPTED MANUSCRIPT Basque Country (UFI 11/39). Idoia Hita is grateful for her Basque Government PreDoctoral Grant (BFI2010-223).

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Graphical abstract

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Highlights Activated carbons prepared with H3PO4 are potential hydrocracking supports P-containing activated carbons show stable C-O-PO3 and C-P-O3 acid sites Carbon-supported Pt-Pd catalyst is able to remove 90 % sulfur and 93 wt% gasoil Deactivating coke is deposited on the whole catalytic surface causing pore blockage

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   